# Allow params/metrics to be declared from a JSON field path

Follow-up to the JSON log ingestion design decision (repowise decision fdb47034ad954e3ea0b898fdc2750c32).

Extend param/metric declaration so a param can be extracted from a JSON field path (e.g. `foo.bar`) on lines that parse as JSON objects, as an alternative to the existing regex-capture-group extraction.

## Acceptance criteria
- `RegexToMetricPrimitiveMatcher` (src/data_value.rs) or a sibling type supports a JSON-field-path extraction source alongside the existing regex-capture-group source, reusing `ParamRole`/`MetricPrimitiveKind`.
- The params declaration UI (Params.tsx) lets a user choose "JSON field" as an alternative to "regex capture group" when declaring a param, for lines detected as JSON.
- Existing regex-based params/metrics are unaffected.